Mikey Borup offers a bit of insights into using the After Effects Property Group Index in expressions. With that expression, you can get the index of a property within your layer, such as a shape layer.
The Property Group Expression essentially lets you target a group of properties relative to the property on which the expression happens to be. Using the Property Group Index, method you can set up a name-independent relationship in the property hierarchy. This method is especially useful when duplicating properties that contain expressions.
